FTP Extension SSL issues + refactor/cleanup of UI logic

Description

As discussed last night on #wordpress-dev, There exists cases where people are getting invalid username/password when using the FTP Extension (Whilst the credentials are correct).

__lance__ tracked it down to SSL being selected, when the UI wasnt showing it as such.. It seems it was coming from $opt['ssl'], which AFAIK, Isnt actually set by WordPress at all (It could've been left over from an early revision..)

The attached patch cleans up some of the UI Logic as well, and takes into consideration FS_METHOD being set, which forces it in some cases.

Also, Includes an uncommited change, a class_exists() around the FS Abstraction file, To check if the class exists already before including a abstraction file, Its mainly for use when a plugin adds a 3rd-party FS Method file (Like my debug trace plugin does, No-wonder it never worked for anyone else)